Setting/getting values through pointers in C++

Question

I'm trying to learn how pointers work in C++, except for the last hour I've only managed to make it crash.

#include<string>

// a sample structure
typedef struct test_obj_type_s{

    // with some numbers
    int x; 
    float y;
    double z;

    std::string str; // a standard string...
    char * str2; // a variable-length string...
    char str3[2048]; // a fixed-length string...

    struct test_obj_type_s *next; // and a link to another similar structure
} test_obj_type_t;

// a sample pointer
test_obj_type_t * last_test_obj;
test_obj_type_t obj;

// let's go
int main(){

    // let's assign some demo values
    obj.x = 12;
    obj.y = 15.15;
    obj.z = 25.1;
    obj.str = "test str is working";
    obj.str2 = "test str2 is working";
    strcpy_s(obj.str3, "test str3 is working"); 

    // let's also assign some circular references
    obj.next = &obj; 

    // now...
    last_test_obj = &obj;

    test_obj_type_t * t1 = last_test_obj;
    test_obj_type_t t2 = obj;

    // now let's have some fun;
    printf("%d %d %d %s %s %s", t2.x, t2.y, t2.z, t2.str, t2.str2, t2.str3);
    printf("%d %d %d %s %s %s", t2.next->x, t2.next->y, t2.next->z, t2.next->str, t2.next->str2, t2.next->str3);
    printf("%d %d %d %s %s %s", t1->x, t1->y, t1->z, t1->str, t1->str2, t1->str3);
    printf("%d %d %d %s %s %s", t1->next->x, t1->next->y, t1->next->z, t1->next->str, t1->next->str2, t1->next->str3);

    printf("I survived!");
}

What am I missing? Or what am I doing wrong?

<< from iostream, and you'll get rid of your crash, since I believe you have an answer. — senseiwa
– senseiwa, Commented Aug 17, 2014 at 14:00
"I'm trying to learn how pointers work in C++, except for the last hour I've only managed to make it crash." - and it will stay like this for the rest of your career. Because even experienced senior software developers create pointer bugs. — Christian Hackl
– Christian Hackl, Commented Aug 17, 2014 at 14:45

Igor Tandetnik · Accepted Answer · 2014-08-17 13:51:05Z

4

Your program exhibits undefined behavior when it passes an std::string to printf. %s format specifier expects a const char* as a parameter. Everywhere you have something.str or something->str in your series of printf statements, replace it with something.str.c_str()

Similarly, %d expects int as a parameter, but you are passing double. To print something.y and something.z, use %f specifier instead.

answered Aug 17, 2014 at 13:51

Igor Tandetnik

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Comments

ars · Accepted Answer · 2014-08-17 14:08:44Z

1

You can move to the c++ stream output, it's more simple way to do such things:

// now let's have some fun;
std::cout<<t2.x<<t2.y<<t2.z<<t2.str<<t2.str2<<t2.str3<<std::endl;
std::cout<<t2.next->x<<t2.next->y<<t2.next->z<<t2.next->str<<t2.next->str2<<t2.next->str3<<std::endl;
std::cout<<t1->x<<t1->y<<t1->z<<t1->str<<t1->str2<<t1->str3<<std::endl;
std::cout<<t1->next->x<<t1->next->y<<t1->next->z<<t1->next->str<<t1->next->str2<<t1->next->str3<<std::endl;

answered Aug 17, 2014 at 14:08

ars

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1 Comment

Christian Hackl Over a year ago

Just what happens if "%d %d %d %s %s %s" in the OP's answer actually comes from a user-defined language-dependent setting? std::cout and friends cannot always make up for the flexibility of format strings. Boost.Format tries to combine the flexibility of format strings with the type safety of C++.

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